FMD (Free Mounting Device) for Precision Controlled Humidity Experiments

A frustrating challenge faced by crystallographers is the often futile and time consuming effort spent improving the resolution of poorly diffracting crystals for complexes. Most efforts to improve diffraction are carried out in the wet lab and entail modifications to sample preparation, crystal growth, and crystal harvesting. Currently, there are few technologies available to improve crystal diffraction quality at the synchrotron. However, the solvent content in protein crystals varies from 30% to 70%, and experiments have shown that humidity changes are correlated with defined (and reversible) unit cell adjustments. Robert Huber’s group developed a revolutionary technology for MX: the Free Mounting Device (FMD), which can manipulate the humidity environment around protein crystals in a controlled and reproducible manner and then allow measurement of diffraction quality as a function of humidity 1. The SIBYLS beamline has the first publicly available FMD system in the world. An example of a result made possible by the FMD was the determination of the crystal structure of the catalytic α subunit of E. Coli DNA polymerase III to 2.3Å resolution2. The previous best resolution was ~3Å and contained disordered domains which became ordered upon dehydration.

FMD - Main Humidty Control Unit


FMD - “Iron Maiden”

fmd_2.jpg fmd_3.jpg

Here is the FMD Iron Maiden on the microscope stand, ready for crystal harvesting.


… and on the goniometer.


FMD - Control Software

This is a screenshot of the software for programming in precise humidty ramps.



  1. Kiefersauer R, Than ME, Dobbek H, Gremer L, Melero M, Strobl S, Dias JM, Soulimane T, and Huber R “A novel free-mounting system for protein crystals: transformation and improvement of diffraction power by accurately controlled humidity changes” J. Appl. Cryst. (2000). 33, 1223-1230

  2. Lamers MH, Georgescu RE, Lee SG, O’Donnell M, Kuriyan J “Crystal structure of the catalytic alpha subunit of E. coli replicative DNA polymerase III” Cell. 2006 Sep 8;126(5):881-92.
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